.TH std::basic_format_string,std::format_string,std::wformat_string 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::basic_format_string,std::format_string,std::wformat_string \- std::basic_format_string,std::format_string,std::wformat_string

.SH Synopsis
   Defined in header <format>
   template< class CharT, class... Args >                           \fB(1)\fP \fI(since C++20)\fP
   struct basic_format_string;
   template< class... Args >

   using format_string =                                            \fB(2)\fP \fI(since C++20)\fP

       basic_format_string<char, std::type_identity_t<Args>...>;
   template< class... Args >

   using wformat_string =                                           \fB(3)\fP \fI(since C++20)\fP

       basic_format_string<wchar_t, std::type_identity_t<Args>...>;

   Class template std::basic_format_string wraps a std::basic_string_view that will be
   used by formatting functions.

   The constructor of std::basic_format_string performs compile-time format string
   checks
   unless the constructor argument is returned by std::runtime_format
   (since C++26).

.SH Member functions

                 constructs a basic_format_string, raising compile error if the
   constructor   argument is not a format string
                 \fI(public member function)\fP
   get           returns the wrapped string
                 \fI(public member function)\fP

std::basic_format_string::basic_format_string

   template< class T >                                                \fB(1)\fP
   consteval basic_format_string( const T& s );
   basic_format_string( /* runtime-format-string */<CharT> s )        \fB(2)\fP (since C++26)
   noexcept;

   1) Constructs a basic_format_string object that stores a view of string s. If the
   argument is not a compile-time constant, or if it cannot be parsed as a format
   string for the formatting argument types Args, the construction is ill-formed.
   This overload participates in overload resolution only if const T& models
   std::convertible_to<std::basic_string_view<CharT>>.
   2) Constructs a basic_format_string object that stores a view of string s as
   returned by std::runtime_format. It does not perform format string checks upon
   construction.

.SH Parameters

       an object that represents the format string. The format string consists of
         * ordinary characters (except { and }), which are copied unchanged to the
           output,
         * escape sequences {{ and }}, which are replaced with { and } respectively in
           the output, and
         * replacement fields.

       Each replacement field has the following format:

       { arg-id (optional) }               \fB(1)\fP
       { arg-id (optional) : format-spec } \fB(2)\fP

       1) replacement field without a format specification
       2) replacement field with a format specification

                     specifies the index of the argument in args whose value is to be
                     used for formatting; if it is omitted, the arguments are used in
       arg-id      - order.

                     The arg-id s in a format string must all be present or all be
                     omitted. Mixing manual and automatic indexing is an error.
   s -               the format specification defined by the std::formatter
       format-spec - specialization for the corresponding argument. Cannot start with
                     }.

         * For basic types and standard string types, the format specification is
           interpreted as standard format specification.
         * For chrono types, the format specification is interpreted as chrono format
           specification.

         * For range types, the format specification is interpreted as
           range format specification.
         * For std::pair and std::tuple, the format specification is
           interpreted as tuple format specification.                     (since C++23)
         * For std::thread::id and std::stacktrace_entry, see thread id
           format specification and stacktrace entry format
           specification.
         * For std::basic_stacktrace, no format specifier is allowed.

         * For std::filesystem::path, see path format specification.   (since C++26)

         * For other formattable types, the format specification is determined by
           user-defined formatter specializations.

std::basic_format_string::get

   constexpr std::basic_string_view<CharT> get() const noexcept;

   Returns the stored string view.

.SH Notes

   The alias templates format_string and wformat_string use std::type_identity_t to
   inhibit template argument deduction. Typically, when they appear as a function
   parameter, their template arguments are deduced from other function arguments.

 template<class... Args>
 std::string format(std::format_string<Args...> fmt, Args&&... args);

 auto s = format("{} {}", 1.0, 2);
 // Calls format<double, int>. Args are deduced from 1.0, 2
 // Due to the use of type_identity_t in format_string, template argument deduction
 // does not consider the type of the format string.

.SH Example

    This section is incomplete
    Reason: no example

   Defect reports

   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.

     DR    Applied to       Behavior as published              Correct behavior
   P2508R1 C++20      there's no user-visible name for  the name basic_format_string is
                      this facility                     exposed

.SH Category:
     * Todo no example
